1. Field of the Invention
The present invention relates to ultrasonic sensors. In particular, the present invention relates to an ultrasonic sensor which includes a piezoelectric element and input-output terminals electrically connected to the piezoelectric element and is used, for example, as a corner sonar or a back sonar of a car.
2. Description of the Related Art
An ultrasonic sensor is a sensor that uses ultrasonic waves to perform sensing. The ultrasonic sensor intermittently transmits ultrasonic pulse signals and receives reflected waves from surrounding obstacles to detect an object. For example, the ultrasonic sensor is used in a car as a corner sonar, a back sonar, or a parking spot sensor that detects a space between the car and an obstacle, such as a side wall, in parallel parking.
An ultrasonic sensor of this type is disclosed in Japanese Unexamined Patent Application Publication No. 2007-318742. FIG. 1 is a cross-sectional view of an ultrasonic sensor 10 disclosed in this document. The ultrasonic sensor 10 includes a case 12 having a bottom portion 14 and a tubular portion 16. The bottom portion 14 is made of metal, such as aluminum, and has a closed surface. The tubular portion 16 is made of metal, such as zinc, and fitted into and bonded by an adhesive to the bottom portion 14.
A piezoelectric element 18 is bonded, by a conductive adhesive, to an inner bottom of the bottom portion 14 of the case 12.
First and second input-output terminals 20 and 22 made of metal are electrically connected to the piezoelectric element 18. The first and second input-output terminals 20 and 22 are extracted from the inside to the outside of the case 12. The first input-output terminal 20 is electrically connected to an electrode on an upper principal surface of the piezoelectric element 18. The first input-output terminal 20 includes a spring terminal 20a having spring properties, a middle portion 20b, and a pin-like extraction portion 20c. 
The second input-output terminal 22 is electrically connected, through the case 12, to an electrode on a lower principal surface of the piezoelectric element 18. The second input-output terminal 22 includes a connection portion 22a, a middle portion 22b, and an extraction portion 22c. 
The first and second input-output terminals 20 and 22 are supported by a substantially columnar support member 24 made of insulating synthetic resin. The first and second input-output terminals 20 and 22 are embedded, at their middle portions 20b and 22b, in the support member 24 and secured to be integral with the support member 24.
In the interior of the case 12, the support member 24 is placed adjacent to the upper principal surface of the piezoelectric element 18 and secured to the tubular portion 16 of the case 12.
In the case 12, a damping member 26 is disposed on the closed surface where the piezoelectric element 18 is placed. In the interior of the case 12, an opening side of the support member 24 is sealed with an expandable filler (not shown).
In the ultrasonic sensor 10 of related art illustrated in FIG. 1, the support member 24 that supports the first and second input-output terminals 20 and 22 is directly attached to the tubular portion 16 that vibrates. As a result, vibrations of the tubular portion 16 are transmitted to the first and second input-output terminals 20 and 22 and vibrate a substrate on which the first and second input-output terminals 20 and 22 are mounted (hereinafter referred to as “vibration leakage”). This vibration leakage causes a long reverberation time (i.e., deteriorates reverberation characteristics). If reverberation is prolonged in detection of a nearby object, a reflected signal is received while reverberation of a transmission signal (burst wave) continues. This makes it difficult to detect a nearby object. Additionally, in the structure illustrated in FIG. 1, where a boundary between the bottom portion 14 and the tubular portion 16 is exposed to a side face of the case 12, it is necessary to take measures to prevent entry of moisture and corrosion under high humidity conditions.
An object of the present invention is to provide an ultrasonic sensor which has reverberation characteristics improved by preventing vibration leakage.